This invention relates to systems that address and apply voltages across selected ohmic heating elements in a matrix.
Each heating element in a thermal printhead or a thermal-ink-jet printhead can have interconnect and drive circuitry dedicated exclusively to it. Alternatively, the heating elements can be configured into a matrix in which the heating elements share the interconnect and drive circuitry. FIG. 1 shows an example of such a prior-art matrix (20). The resistors in each row share drive circuitry (26) and the resistors in each column share electrical ground (28). If an individual resistor is "addressed" (i.e., selected), the drive voltage is applied to its row connector (29) and its column connector (30) is grounded, thus creating a voltage drop across it and causing it to dissipate electric power as heat. However, if a row or column does not include an addressed resistor, then the corresponding row connector (29) or column connector (30) is disconnected from drive circuitry (26) or electrical ground (28), respectively, and will assume a voltage imposed by other parts of the circuit.
If an independent voltage source or electrical ground is directly connected via low-resistance conductors to each end of a resistor, thus establishing the voltage across it, then the resistor will be said to be "directly driven". Addressed resistors are directly driven at full power. In the circuit shown in FIG. 1, only addressed resistors (24) are directly driven and "parasitic" voltages appear across unaddressed resistors (22, 23, 25, 27). The parasitic voltages result from current flowing through unaddressed resistors along alternate paths between the drive voltage source and electrical ground (e.g., through the combination of unaddressed resistors (23), (25), and (27)). These currents are referred to as "parasitic currents".
Although power dissipation is desired in the addressed resistors only, the parasitic voltages cause significant power dissipation in the unaddressed resistors. The magnitude of the parasitic voltage across any particular resistor is influenced by the dimensions of the matrix, the number and location of the addressed resistors, and other factors. The total power dissipation of all the resistors in a matrix depends on the number of addressed resistors and the magnitudes of the parasitic voltages across the unaddressed resistors. If, as in standard applications, a resistor matrix energizes arbitrary addressable patterns of resistors, the parasitic voltages may become excessive and the total power dissipation will vary greatly.
Two problems arise when the prior-art resistor matrices are used in thermal printheads or thermal-ink-jet printheads. First, the parasitic voltages across the unaddressed resistors may become large enough to cause the printhead to misfire. For example, in the prior-art matrix (20) shown in FIG. 1, the voltage across some of the unaddressed resistors can reach two-thirds of the drive voltage, resulting in a power dissipation that is 4/9 of the power dissipated by an addressed resistor. This unwanted power dissipation is likely to be of sufficient magnitude to cause the printhead to misfire. Second, the size of the ink droplets ejected by a thermal-ink-jet printhead or the size of the dots printed by a thermal printhead depends on the printhead temperature which generally depends on the total power dissipation of all the heating elements. If the total power dissipation varies appreciably during operation, the resulting printhead temperature variations can cause non-uniformity in the size of the printed dots and degrade the print quality.
U.S. Pat. No. 4,791,440, by Eldridge et al. entitled Thermal Drop-On-Demand Ink Jet Print Head, discloses a resistor matrix that solves some of the problems of prior-art matrices by directly driving some, but not all, of the unaddressed resistors. The remaining unaddressed resistors are not directly driven and parasitic voltages appear across them. As described above, the magnitudes of the parasitic voltages are determined by operating conditions and various parameters of the matrix, rather than being set by the drive circuitry. The maximum ratio of the power dissipated by an unaddressed resistor to the power dissipated by an addressed resistor is 4/9, 9/16, or 16/25 for Eldridge matrices having 3, 4, or 5 rows respectively. The power dissipated by individual unaddressed resistors and the variations in the total power dissipation can become excessive and cause the problems described above.